1. Field of the Invention
The invention relates generally to disk drives, and more particularly to a magnetic recording load/unload type of disk drive that unloads the recording heads when disk drive power is removed.
2. Description of the Related Art
Magnetic recording hard disk drives (HDDs) are information storage devices that use rotatable disks with concentric data tracks containing the information, a head or transducer for reading and/or writing data onto the various tracks of each disk surface, and an actuator for moving the heads. Each head is located on a head carrier and each carrier is connected to the actuator by a suspension. The actuator is a voice coil motor (VCM) comprising a coil movable through a magnetic field generated by a fixed permanent magnet assembly. The HDD has a servo control system that receives a position error signal (PES) from servo positioning information read by the heads from the data tracks and generates a VCM control signal to maintain the heads on track (track “following”) and move them to the desired track (track “seeking”) for reading and writing of data. The disks are stacked on a hub that is rotated by a disk drive motor, also called a spindle motor. A housing supports the spindle motor and actuator, and surrounds the heads and disks to provide a substantially sealed environment for the head-disk interfaces.
The head carrier is typically an air-bearing slider that rides on a bearing of air above the disk surface when the disk is rotating at its operational speed. The slider is maintained next to the disk surface by a suspension that connects the slider to the actuator. The slider is either biased toward the disk surface by a small spring force from the suspension, or is “self-loaded” to the disk surface by means of a “negative-pressure” air-bearing surface on the slider.
In a “load/unload” type of HDD, the sliders are mechanically unloaded from the disks when power is turned off, and then loaded back to the disks when the disks have reached a speed sufficient to generate the air bearing. The loading and unloading is typically done by means of ramps that contact the suspensions when the actuator is moved away from the data regions of the disks. Each slider is thus “parked” off its disk surface with its suspension, or a tap extending from the suspension, supported in a recess of the ramp. Load/unload HDDs provide a benefit in laptop computers because the parking of the sliders on the ramps away from the disk surfaces also provides some resistance to external shocks caused by moving or dropping the computer.
The parking of the sliders on the load/unload ramps when HDD power is removed is typically accomplished by use of the back electromotive force (EMF) generated by the freely rotating spindle motor. When the HDD is powered down, or in the event of unexpected loss of power (an emergency power-off or EPO event), actuator retract circuitry disconnects the VCM from its driver circuitry and connects it to a rectifier circuit that is coupled to the spindle motor. The output of the freely-rotating spindle motor is converted by the rectifier circuit to a DC current supplied to the coil of the VCM. This causes the VCM to retract to move the sliders to the ramps. A significant amount of torque is needed to ensure that the sliders are fully parked on the ramp, regardless of the VCM position or velocity at power down or EPO. The actuator velocity during retract needs to be controlled to avoid the sliders hitting the ramps at high speed. Excessive slider motion can cause the sliders to contact the ramp structure, or perhaps other sliders. Such contact can result in slider damage or transfer of contamination to the air-bearing surface, which can lead to head-disk interface failures.
In normal power down, the movement of the actuator can be controlled by software that controls the retract circuitry. However, during EPO, in which power to the HDD is lost unexpectedly, the software control is not active. The current supplied to the VCM during EPO retract must be large enough to unload the sliders to the ramps but small enough to prevent the suspensions and sliders from impacting the ramps at excessive speed. The EPO retract is acceptable if EPO occurs when the actuator is track following and thus has no initial velocity. However, safe retract becomes problematic if EPO occurs during track seeking, when the actuator is moving. The actuator seek velocity depends on the seek length, and can be considerably high, so that the actuator retract velocity can be significantly increased due to the initial velocity of the actuator. To address this problem, if EPO occurs during a track seek HDDs use dynamic braking of the VCM to release residual energy inside the VCM to ground before initiating actuator retract. The VCM dynamic braking is designed to brake the actuator for short track seeks, when the actuator velocity is low. For medium and long track seeks from outside diameter (OD) to inside diameter (ID) direction, the dynamic braking may be too weak so that the actuator impact speed on the ID crash stop is still high and later causes the sliders to stall at the ramps or rebound back from the ramps towards the disks. The stalling and rebounding of the sliders can cause damage to the heads and disks. For medium and long track seeks from ID to OD direction, the dynamic braking is not strong enough to avoid high speed impact during unloading. High speed impact between the sliders and the ramps also can cause damage to the heads and disks.
What is needed is an HDD with a reliable method for braking the actuator if EPO occurs during a track seek.